1. Technical Field
The disclosure relates in general to a receiver, and more particularly to a receiver with a wide-dynamic-range variable gain.
2. Description of the Related Art
In the field of telecommunication, the receiver for receiving signals normally uses a dynamic amplifier to amplify the received signal. When the amplitude of the received input signal changes, the gain of the dynamic amplifier also changes accordingly to maintain the intensity of the output signal.
FIG. 1 is a circuit diagram showing a conventional dynamic amplifier utilizing a current steering circuit. Referring to FIG. 1, the dynamic amplifier 100 uses a current steering circuit 140 to adjust the gain of the dynamic amplifier 100 according to the voltage of a control signal VCTL. When the amplitude of the input signal Vi raises, the voltage of the control signal VCTL lowers so as to reduce the magnitude of the output current io, thereby reducing the gain of the dynamic amplifier 100. However, when the amplitude of the input signal Vi is too large, the transistors Q121 and Q122 Of the differential amplifier 120 are operated within a saturation region, making the output signal distorted. Despite the magnitude of the output current io is reduced, the distortion problem in the waveform of the output current io still remains unresolved. Thus, the conventional dynamic amplifier 100 has the disadvantage that the level of linearity still cannot meet the requirement. Here, linearity refers to the range of amplitude of the input signal which does not generate a distorted output signal.
In order to increase the level of linearity, a conventional dynamic amplifier with degenerating resistance tuning is provided, and the circuit diagram of a degenerating resistance circuit is shown in FIG. 2. The degenerating resistance tuning is implemented by a metal oxide semiconductor field-effect transistor (MOS transistor). The dynamic amplifier 200 controls the control voltage VCTL′ at the gate of the field-effect transistor 240 to change the equivalent resistance of the conducted field-effect transistor 240 so as to adjust the gain of the dynamic amplifier 200. Thus, the range of amplitude of the receivable input signal which does not make the dynamic amplifier 200 generate a distorted output signal is increased, so as to increase the dynamic range of the dynamic amplifier 200. The gain of the dynamic amplifier 200 is related to the impedance value of the field-effect transistor 240. When the frequency of the input signal Vi is too high, the parasitic capacitance of the degenerating resistance circuit affects the gain more seriously, so that the gain of the dynamic amplifier 200 cannot be precisely controlled when the frequency is too high. Meanwhile, the linearity of the dynamic amplifier 200 is also hard to control when receiving a high-frequency input signal.
To resolve the above control problem of the linearity of the dynamic amplifier 200, a wide-dynamic-range variable-gain amplifier circuit is disclosed in U.S. Pat. No. 6,049,251. As indicated in FIG. 3A, a circuit diagram of a wide-dynamic-range variable-gain amplifier circuit according to U.S. Pat. No. 6,049,251 is shown. The wide-dynamic-range variable-gain amplifier 300 includes two amplifier circuits 320 and 340. FIGS. 3B and 3C respectively show circuit diagrams of the amplifier circuit 320 and the amplifier circuit 340 of FIG. 3A. Referring to FIG. 3B, the resistor RE1 has smaller resistance, so the amplifier circuit 320 has large gain but low level of linearity. Referring to FIG. 3C, the resistor RE2 has larger resistance, so the amplifier circuit 340 has small gain but high level of linearity. The amplifier circuits 320 and 340 are coupled in parallel, and the gains of the amplifier circuits 320 and 340 are respectively controlled by the automatic gain control signals AGC1 and AGC2. As the amplifier circuits 320 and 340 have different gains, so the amplifier with two different levels of linearity can be implemented and the wide-dynamic range can be achieved. However, in order to increase the range of the acceptable input signal, at least one more amplifier circuit needs to be coupled in parallel, not only increasing the required elements and cost but also increasing the complexity of the entire circuit.
Moreover, a discrete automatic gain control mechanism is disclosed in U.S. Pat. No. 5,161,170. Despite the discrete automatic gain control mechanism adopts discrete digitization technology to avoid using huge and expensive analog elements as many as possible, some analog elements are still necessary in order to lock the output signal power to a constant level. Thus, an automatic gain control mechanism for digitized RF signal processing is disclosed in U.S. Pat. No. 7,088794. FIG. 4 shows a circuit diagram of a digital automatic gain control signal controller according to U.S. Pat. No. 7,088,794. Referring to FIG. 4, the digital automatic gain control signal controller 400 utilizes an analog-to-digital converter 440 to convert an analog input signal into a digital signal. Then, a signal detection logic unit 460 judges the level of the digital signal and accordingly controls a signal detection logic unit 460 by feedback control to determine the attenuation level of the analog input signal.
In addition, an automatic gain control system is also disclosed in U.S. Pat. No. 7,085,334. The automatic gain control system uses an analog-to-digital converter to convert an analog input signal into a digital signal. Then, a post-stage digital signal processing (DSP) circuit operates the digital signal to accordingly control the analog gain device to receive the analog input signal by feedback control. Moreover, a mixed analog-digital automatic gain control (AGC) circuit is disclosed in U.S. Pat. No. 7,382,296. The mixed analog-digital AGC circuit also utilizes an analog-to-digital converter to convert an analog the input signal into a digital signal, which a post-stage circuit determines and controls the gain of an analog amplifier by feedback control according to. However, the above technologies of converting an analog input signal into a digital signal have to employ a multi-bit analog-to-digital converter, and are inconvenient in the sampling when the input signal is a high-frequency signal. Thus, the above circuits are hard to implement.
Therefore, how to increase the range of the input signal acceptable to the amplifier without increasing circuit complexity has become a focus to the manufacturers.